Ultrasonic harmonic imaging system and method

ABSTRACT

A method and system for ultrasonically imaging a target with energy spreading transmissions is provided. Simultaneous and sequential compound point, line and combination point and line focusing is used with harmonic receive processing. Ultrasonic energy focused at multiple depths is transmitted at the target. The target may or may not include contrast agents. In either case, echoes of the transmissions at one or more fundamental center frequencies are received at harmonics of the fundamental frequencies.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/081,918, (later abandoned) filed May 19, 1998, which is inturn a continuation-in-part of U.S. patent application Ser. No.08/893,288, now U.S. Pat. No. 6,005,827, filed Jul. 15, 1997, which isin turn a continuation-in-part of U.S. patent application Ser. No.08/642,528, filed May 3, 1996, now U.S. Pat. No. 5,740,128, which is inturn a continuation-in-part of U.S. patent application Ser. No.08/397,833, filed Mar. 2, 1995, now U.S. Pat. No. 5,608,690. Theseapplications and patents are hereby incorporated by reference in theirentirety.

BACKGROUND

This invention relates to ultrasound imaging systems, and in particularto improved methods for imaging nonlinear contrast agents and tissuewith such systems.

Nonlinear contrast agents are described for example by V. Uhlendorf, etal., in "Nonlinear Acoustical Response of Coated Microbubbles inDiagnostic Ultrasound" (1995 Ultrasonic Symposium, pp. 1559-1562). Suchagents possess a fundamental resonant frequency. When they areinsonified with high intensity ultrasonic energy at this fundamentalfrequency, they radiate ultrasonic frequency at a harmonic of thefundamental frequency. Such contrast agents are often used to highlightregions containing blood loaded with the contrast agent. For example, inthe case of a blood-filled chamber of the heart, the borders of thechamber can be distinguished more easily when contrast agent is used.Since the contrast agent generates harmonic ultrasound energy, echoesfrom tissue (containing no contrast agent) at the fundamental frequencymay be eliminated by filtering at the receive beamformer.

Typically, such agents are used with an imaging system having a transmitbeamformer that transmits ultrasonic energy at the fundamental frequencyand a receive beamformer responsive to the harmonic. In order to imagethe contrast agent clearly, it is known to reduce energy at the harmonicin the transmit beam, and to reduce sensitivity of the receivebeamformer to energy at the fundamental.

In the past, this has been done by using a burst of square or sine wavesto form the transmit beam, and by using appropriate band pass or highpass filters in the receive beamformer. Though a large pulse countreduces energy at the harmonic, it reduces time resolution of the pulse,and therefore spatial resolution of the resulting image.

The present invention is directed to further improvements that enhancethe imaging of such nonlinear contrast agents, as well as the nonlinearresponse of tissue.

SUMMARY

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. By way ofintroduction, the system described below ultrasonically images a targetwith energy spreading transmissions. Generally, ultrasonic energycorresponding to a multiple depth focus is transmitted into a target ata fundamental center frequency. The target may or may not includecontrast agents. In either case, echoes are received at a harmonic ofthe fundamental center frequency.

In one mode of operation, ultrasonic energy centered at a fundamentalfrequency and focused at a first depth is transmitted along a scan line.Substantially simultaneously, ultrasonic energy centered at thefundamental frequency and focused at a second depth is transmitted alongthe scan line. Ultrasonic energy from the target is received at aharmonic of the fundamental frequency and is associated with at leastthe first and second depths. In alternative modes of operation, thefirst and second depths are associated with different scan lines.

In a second mode of operation, the target is free of contrast agentthroughout an entire imaging session. Ultrasonic energy centered at afundamental frequency and focused at a first depth is transmitted alonga scan line. Ultrasonic energy from the target is received at a harmonicof the fundamental frequency and is associated with at least the firstdepth. After transmitting and receiving at the first depth, the transmitand receive steps are repeated for a second depth.

In a third mode of operation, sequential transmission and reception asdiscussed above is performed, regardless of the existence of contrastagent in the target. The process is repeated for a plurality of scanlines. A frame of data associated with the plurality of scan lines isgenerated.

In a fourth mode of operation, a first transmit beam focused at a firstdepth is transmitted, and a second transmit beam having a line focus isalso transmitted. In other modes of operation, two transmit beams eachhaving a line focus are transmitted at a fundamental center frequency.Echo information associated with harmonic frequencies are received.

The transducer is preferably tuned or optimized for transmission ofwaveforms at a fundamental frequency and receive of waveforms at aharmonic frequency is provided.

Further aspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ultrasonic imaging system suitable foruse with the method of this invention.

FIG. 2 is a block diagram of a transmit beamformer suitable for use inthe system of FIG. 1.

FIG. 3 is a graph of a Gaussian pulse in the frequency domain.

FIG. 4 is a graph of a waveform corresponding to the Gaussian pulse ofFIG. 3 in the time domain.

FIG. 5 is a graph of a focusing profile suitable for an axicon focus.

FIG. 6 is a schematic representation of a compound focus arrangement.

FIG. 7 is a graph showing high and low frequency wavefronts.

FIG. 8 is a block diagram of a second transmit beamformer suitable foruse in the system of FIG. 1.

FIG. 9 is a block diagram of a third transmit beamformer suitable foruse in the system of FIG. 1.

FIGS. 10, 11, and 11a are block diagrams of portions of transmitbeamformers that incorporate preferred embodiments of this invention.

FIG. 12 is a waveform diagram illustrating operation of the beamformersof FIGS. 10-11a.

FIGS. 13-16 are graphs illustrating the temporal and frequencycharacteristics of waveforms produced by the beamformers of FIGS.10-11a.

FIG. 17 is a graph illustrating a filter transfer function suitable foruse in the beamformers of FIGS. 10-11a.

FIGS. 18-23 are block diagrams of alternative filter circuits suitablefor use with ultrasonic harmonic imaging systems.

FIG. 24 is a circuit diagram of one embodiment for tuning a transducer.

FIG. 25 is a circuit diagram of another embodiment for tuning atransducer.

FIG. 26 is a graph illustrating the frequency response of a tunedtransducer.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The preferred embodiments described below are designed to reduceharmonic energy in the transmitted beam, and to provide an improvedspatial distribution of fundamental energy in the transmitted beam. FIG.1 shows generally an ultrasonic imaging system 10 which can be used topractice the method of this invention.

The system 10 includes a transmit beamformer 12 that supplies highvoltage transmit waveforms via a multiplexer 14 to a transducer array16. The transducer array 16, which can be any suitable type, generatesan ultrasonic transmit beam in response to the transmit waveforms, andthis transmit beam propagates outwardly through the subject 18 beingimaged. In this case, the subject being imaged includes a nonlinearcontrast agent 20, such as that described above. As used herein,nonlinear contrast agent refers to any contrast agent with a nonlinearresponse, whether the contrast agent is designed for fundamental orharmonic imaging. Any suitable contrast agent may be used, as long as itabsorbs ultrasonic energy at a first frequency and radiates ultrasonicenergy at a second frequency, different from the first frequency whensubject to a high intensity acoustic pulse. In this example, the firstfrequency is referred to as the fundamental frequency, and the secondfrequency is a harmonic of the first frequency. As used herein,"harmonic" is intended broadly to include sub-harmonics and fractionalharmonic energy (e.g. 1/2 or 3/2 of the fundamental), as well as higherharmonics (e.g. 2 or 3 times the fundamental).

Ultrasonic energy radiated by the nonlinear contrast agent 20 at theharmonic frequency is received by the transducer array 16, focused bythe receive beamformer 22, and displayed as an image by a displayprocessor (not shown). As described above, the receive beamformer 22includes conventional filters to substantially prevent reflectedultrasonic energy at the fundamental frequency from being imaged.

Turning now to FIG. 2, this figure shows a block diagram of a firstpreferred embodiment 40 of the transmit beamformer of FIG. 1. As shownin FIG. 2, the transmit beamformer 40 includes N channels, one for eachof the transducers of the transducer array 16 (FIG. 1). Each channelincludes a delay memory 42, a waveform memory 44, and a delay counter 46(FIG. 2). The delay memory 42 includes 256 words 48, one for eachpossible steering angle or ultrasound transmit scan line. Each word 48of the delay memory 42 is set equal to a negative number equal to thenumber of clock cycles on the clock signal line 50 that elapse between astart of line signal on line 54 and the first non-zero value of theassociated waveform. For simplicity, it is assumed that zero is definedas a word 48 having the most significant bit equal to one and all otherbits equal to zero. Hence, the most significant bit becomes an enablesignal for the waveform memory 44.

The waveform memory 44 in this embodiment stores a single waveform indigital form, which is used for all transmit scan lines. The waveformmemory 44 can include for example 64 or 128 successive 8 bit words. Themagnitude of each 8 bit word corresponds to the voltage amplitude at therespective position in the waveform. When the waveform memory 44 is readwith a 40 MHz clock on the line 50, the resulting sequence of digitalvalues defines a waveform approximately 1.6 to 3.2 microseconds induration.

The delay memory 42 is not required, but it reduces memory requirementsfor the waveform memory 44. This is because the delay memory 42eliminates the need to store a large number of leading zeros when theultrasound line is steered at a large angle.

In use, each channel responds to a scan line selection signal on line 52by loading the word 48 for the selected scan line into the delay counter46. The delay counter 46 responds to a start of scan line signal on line54 by incrementing the stored value with each cycle of the 40 MHz clockon line 50. When the counter 46 increments to zero, it enables thewaveform memory 44. Subsequently generated values of the counter 46(incrementing now from zero upwards) become address values for thewaveform memory 44. As each word of the waveform memory 44 is addressed,the corresponding 8 bit word is read and applied to a digital to analogconverter 56.

The analog output signal of the converter 56 is passed through a lowpass filter such as a Bessel filter 58 to reduce sampling effects andthen to an amplifier 60. The output of the amplifier 60 can be passedthrough an additional low pass filter 62 to improve harmonic rejection.The output of the low pass filter 62 is the transmit waveform discussedabove that is applied to the respective transducer via the multiplexer14 (FIG. 1). The low pass filters 58, 62 preferably provide a sharpcut-off with a low stop band level in order substantially to eliminateultrasonic energy in the transmitted pulse at the harmonic frequency.

The transmit beamformer 40 utilizes values for the waveforms stored inthe waveform memory 44 and the delays stored in the delay memory 42 thatenhance insonification of the nonlinear contrast agent in the subject.

The waveform stored in the waveform memory 44 is shaped to suppressultrasonic energy in a wide pass band centered at the harmonicfrequency. For example, the spectrum of the desired pulse can bedesigned on a computer 64. FIG. 3 shows the frequency spectrum of onesuitable pulse 70 which is centered at the fundamental frequency of 2.5MHz and is generally Gaussian in shape. The particular Gaussian shapeshown in FIG. 3 has an amplitude reduced by 71 dB at 3.71 MHz. Thebandwidth of the pulse 70 is 30% of the center frequency, measured atpoints -6.8 dB with respect to the peak amplitude. Such a pulse will bereferred to herein as a 30% BW Gaussian pulse. Note that the pulse 70has substantially no energy at 5 MHz, the first harmonic of thefundamental frequency. This invention is not limited to use withGaussian pulses, and a wide range of spectra can be used.

Once the desired pulse has been designed, an inverse fast Fouriertransform is then performed to generate the corresponding time domainwaveform. FIG. 4 shows a waveform 80 which corresponds to the pulse 70of FIG. 3. Note that the waveform 80 includes an oscillating componenthaving a frequency of about 2.5 MHz. This oscillating component isshaped by an envelope 82. The envelope 82 rises gradually from zeroamplitude to a maximum amplitude, and then falls gradually from themaximum amplitude back to zero amplitude. Thus, the envelope 82 is quitedifferent from the envelope for a switched pulse train, which issubstantially rectangular in shape. The gradually increasing andgradually decreasing envelope 82 of FIG. 4 brings with it the advantageof reduced ultrasonic energy at harmonics of the fundamental frequency.

Once a waveform such as the waveform 80 of FIG. 4 has been designed, thewaveform 80 can be coded into binary samples at a suitable sample rateand then stored in the waveform memory 44. The waveform memory 44 may bea read only memory, in which case the computer 64 may not be required tobe connected to the transmit beamformer 40. Alternately, the waveformmemory 44 may be a volatile memory which is programmed at power-upinitialization by the computer 64. The computer 64 may perform anydesired subset of the pulse designing steps described above. Typically,the desired pulse may be one of several selectable pulses included in amenu for user choice.

When the waveform in the waveform memory 44 is designed as describedabove, the result is a broad band waveform in the waveform memory 44which simultaneously has substantially no radiated energy in a broadband centered on the harmonic. In the example of FIGS. 3 and 4,substantially no ultrasonic energy is radiated at frequencies above 4MHz, or in a bandwidth of □1 MHz with respect to the first harmonic (5MHz). Preferably, the energy component at the harmonic is more than 30dB reduced with respect to the magnitude of the fundamental frequency,and ideally is reduced by more than 40 dB.

Of course, it is not necessary to define the waveform 80 initially inthe frequency domain. A Gaussian pulse can be defined in the timedomain. Furthermore, the envelope need not be Gaussian, it may be someother window function such as a Hamming pulse, a modified Gaussianpulse, or any other suitable pulse. In some applications it may bepreferable to use a narrow bandwidth pulse and thereby achieve a veryhigh reduction of energy at the harmonic, since the harmonic of thelower bandedge is well above the upper bandedge. On other occasions itmay be preferable to use a wider bandwidth pulse, for example, to obtainbetter axial (temporal) resolution. In this case, somewhat reducedreduction of energy at the harmonic may be accepted.

An alternative approach is to design the pulse 70 to ensure thatsubstantially no energy is transmitted above 1.5 times the centerfrequency of the intended fundamental pulse (3.75 MHz in this case). Itis preferable to limit low frequency energy in the transmitted pulse sothat the harmonic energy associated with this frequency does not fallwithin the spectrum of the transmitted pulse. If the cut off frequencyis 3.75 MHz, there should be very little transmitted energy below 1.875MHz.

In designing the pulse 70 and the waveform 80, the Gaussian waveform maybe passed through a high order, low pass filter to eliminate allharmonic energy. This filtering may be done off line in the computer 64.

Optimum imaging of the nonlinear contrast agent is obtained when thetransmit beam insonifies the agent at power levels within a desiredrange. Power levels below this range may not be sufficiently high tocause the nonlinear contrast agent to radiate at the harmonic frequency.Power levels above this range may destroy the contrast agentprematurely. Additionally, since there are FDA limits on ultrasoundintensity, a sharply focused transmit beam is not optimal. Such asharply focused beam provides a high intensity (near the FDA limits) atthe focus, but an intensity that is undesirably low at many other pointsalong the associated receive scan line.

The receive beamformer 22 preferably receives samples along an entirescan line for each transmit event. For this reason, it is preferablethat the region of insonification within the desired power level rangebe extended over a substantial portion of the length of the receive scanline. Thus, it is preferable that the intensity of the transmittedultrasonic energy be substantially uniform and at a high levelthroughout the field of interest (which is typically a large fraction ofthe displayed depth).

The delay memory 42 preferably stores delay values to provide acontrolled spread to the beam intensity in a way to optimize imaging ofthe contrast agent. Also, by making the intensity of harmonic energyreceived at the receive beamformer 22 more uniform throughout the fieldof interest, the levels of harmonic back scatter may be bettercontrolled, resulting in manageable voltage swings at the receiverinput.

In this embodiment, the delay values stored in the delay memory 42 areselectively chosen to spread the beam along the current ultrasound line.One way to accomplish this is to use the well-known axicon focusingarrangement, as described, for example by C. Burckhardt in "UltrasoundAxicon: A Device for Focusing over a Large Depth" (J. Acoust. Soc. ofAm., 54, 6, pp. 1628-1630 (1973)). The axicon focusing arrangement mayutilize a focusing profile as shown in FIG. 5. Typically, this focusingprofile provides a near focal limit corresponding to a circular arccentered on the near focal limit. Typically, the delay profile extendslinearly outwardly from this circular arc to some outer limit, as shownin FIG. 5.

The objective is to spread the ultrasound energy throughout a region ofthe target, and many different delay profiles may accomplish thisresult. For example, the delay profile may be slightly curved, with anonlinear variation of focal point with respect to transducer elementposition. There may be an outer focal limit, in which case the delayprofile can include a circular portion at the ends of the array.

In many applications, it will be desirable to select the delay values inthe delay memory 42 such that at least first frequency components of thetransmit beam from at least a first plurality of transducers are focusedat a first, shorter range, and that at least second frequency componentsof the transmit beam from at least a second plurality of transducers arefocused at a second, longer range. One example is shown in FIG. 6, wheresubstantially all of the ultrasonic energy from the transducers 16a atthe end portions of the transducer array 16 are focused at a singlelonger range X1, and substantially all of the ultrasonic energy from thetransducers 16b at central portions of the array are focused at asingle, shorter range X2. By properly selecting the delay values, a linefocus or a multiple-point compound focus may be obtained. When a linefocus is used, the line may be straight or curved.

Another approach begins with focal delays required for a chosen focalpoint in the conventional manner. A random delay error is superimposedon these focal delays to smear or defocus the resulting beam. Thegreater the degree of defocusing, the more spread out the beam is.Preferably, a user control is provided to allow the user to vary thedegree of defocusing by increasing the relative level of the defocusingdelays. Also, it is preferable to increase the transmitted energy levelto partially compensate for the loss of peak field intensity due todefocusing, either in an automatic (internally programmed) manner orunder user control. The defocusing hardware can consist of a modifiedversion of aberration correction hardware in which the delay correctionsare pre-programmed random numbers rather than values which arecontinuously updated, for example by means of cross-correlating thewaveforms from adjacent elements.

Conventional imaging at the fundamental frequency using an axicon ordefocused beam is known to produce side lobes. However, such side lobesare not anticipated to create substantial problems in this application,particularly if the side lobes are below the harmonic activationthreshold intensity and they therefore do not contribute to thegeneration of harmonic energy. Full dynamic receive focusing ispreferably performed in the receive beamformer 22 to reduce the effectof side lobes in the transmit beam further.

Regardless of the precise configuration of the pulse 70 and the waveform80, the waveform 80 preferably provides more uniform field intensitythrough a usefully extended depth of field. This results in more uniformgeneration of harmonic energy by the contrast agent and possibly ahigher overall signal to noise ratio since more of the field is beingunisonified at a sufficiently high power level to cause the contrastagent to radiate harmonic energy, but at a sufficiently low power levelto prevent unnecessarily rapid destruction of the contrast agent.

If desired, the axicon focus may be obtained with a lens. Also, anaxicon focusing scheme may be used in the elevation direction, if it isdesired to increase the dimension of the insonified region in thatdirection.

The transmit beamformer 40 of FIG. 2 is quite similar to the beamformershown in FIG. 13 of U.S. Pat. No. 5,608,690, and the techniquesdescribed above relating to Gaussian waveforms modified to provide aline focus can be performed in the manner described in theabove-identified application. Note in particular pages 23 and 24 of theabove-identified application as filed, which expressly relate tooptimized transmit beamformers for use with nonlinear contrast agents.With this approach, a plurality of transmit waveforms are provided, eachfor a respective one of the transducers of the transducer array. Thisplurality of transmit waveforms includes a central transmit waveformassociated with a central one of the transducers. As explained in theabove-identified patent application in detail, the central transmitwaveform preferably comprises a lower frequency component and a higherfrequency component, and a lower frequency component of the centraltransmit waveform occurs earlier in time than the higher frequencycomponent of the central transmit waveform, as shown in FIG. 7. FIG. 7corresponds to FIG. 11 of above-identified U.S. Pat. No. 5,608,690, andthat application can be referenced for additional information regardingthese figures. When the system of U.S. Pat. No. 5,608,690 is adapted foruse with this invention, it is preferred that the frequencies along theline focus all be near the fundamental frequency to insonify thenon-linear contrast agent effectively.

Additionally, the transmit beamformer described in Cole, et al., U.S.patent application Ser. No. 08/286,652, (abandoned) filed Aug. 5, 1994,and in U.S. patent application Ser. No. 08/432,056, filed May 2, 1995(now U.S. Pat. No. 5,675,554), both assigned to the assignee of thepresent invention, can be adapted for use with this invention. Once thedesired output is defined as described above in terms of very lowharmonic signal, one can then define the ideal output signal in thefrequency domain and then convert it to the time domain. This timedomain signal can then be divided by the carrier to obtain the desiredenvelope using complex shapes for both the time domain signal and thecarrier. This combination of envelope and carrier can then be programmedinto the transmit waveformer, using the parameters of attachedAppendix 1. Appendix 1 provides parameters for both B-mode and FlowMode. Appendix 3 provides a revised set of transmit beamformerparameters, and Appendix 4 provides a preferred set of transmitbeamformer parameters adapted for use with the Acuson 3V2C transducerprobe.

The envelope is sampled at a relatively low frequency, and as a resultof imperfections in real implementations, remnants of harmonics relatingto the sampling frequency of the carrier and the base band signal mayappear in the final result.

In this example, harmonic energy at the second harmonic from thecontrast agent is imaged using the receive beamformer described inWright, et al. U.S. patent application Ser. No. 08/286,658, (abandoned)filed Aug. 5, 1994, and in U.S. patent application Ser. No. 08/432,615,filed May 2, 1995 (now U.S. Pat. No. 5,685,308), both assigned to theassignee of the present invention. This receive beamformer can beprogrammed using the parameters of attached Appendix 2.

For example, the transmit beamformer of U.S. patent applications Ser.Nos. 08/286,652 and 08/432,056 can be operated with a nominal centerfrequency FO equal to 5.0 MHz, a transmit carrier frequency F, equal to2.5 MHz, and a Gaussian envelope having a 50% bandwidth at -6.8 dB with8/4 envelope sampling.

FIG. 8 shows another transmit beamformer 100 that can be used in theultrasound imaging system 10. The beamformer 100 includes a pulsegenerator 102 which supplies a burst of pulses to a low pass filter 104.In this case, the pulse generator 102 switches between a selectable highvoltage DC level and ground. The duration of the pulses and the numberof pulses in the burst are controllable, as described, for example, inU.S. Pat. No. 4,550,067 (Maslak, et al.), assigned to the assignee ofthe present invention. The low pass filter 104 is designed as describedabove to substantially eliminate energy at the harmonic. The low passfilter 104 can be an analog filter such as a suitable Bessel orChebyshev filter.

FIG. 9 shows another transmit beamformer 120 that can be used in thesystem 10 of FIG. 1. The transmit beamformer 120 includes a pulsegenerator 122 which supplies pulses to a low pass filter 124. The outputof the low pass filter is supplied to a high voltage amplifier 126.Because the low pass filter 124 is applied prior to the amplifier 126,the low pass filter 124 can be designed with smaller components andhence can be higher order and more effective in suppressing harmonicenergy.

It should be noted that conventional transducer arrays have a finitebandwidth, such as 75% at the -6 dB levels. Considering the examplewhere the fundamental frequency is 2.5 MHz and the harmonic frequency is5 MHz, the transducer will typically be centered between the transmitand receive frequencies (as for example at 3.75 MHz), with a relativelyhigh bandwidth. Notice that when this transducer is operated at 2.5 MHzwith a symmetrical band shape, the transducer will skew the transmittedspectrum toward higher frequencies. In the design of the transmittedpulse, the spectrum should preferably be modified to take account of theskewing effect of the finite bandwidth effects of the transducer and thefact that the transducer is working away from its center frequency intransmit.

Of course, the techniques described above can be used in systems 10which provide apodization in the normal manner. If desired, apodizationprofiles can be modified if experience shows that the contributions fromthe end transducer elements (which are focused at deep ranges) are tooattenuated. In this case, it may be preferable to increase the weightingat the end elements to compensate for this effect to some extent. Byoperating the end transducer elements at higher power levels thancentral transducer elements, more nearly uniform power levels areobtained at various ranges in the tissue being imaged.

A further modification is to use fewer transmit lines than receivelines. Since contrast agent tends to be consumed by the insonificationprocess, it is preferable to reduce the number of transmit lines byreducing the frequency of firings and/or the spatial density of lines.Reduced firings and density can be achieved by firing one fourth thenumber of transmit lines and forming four synthetic receive linesclosely spaced around each transmit line, which may be slightlydefocused if required. Wright, et al. U.S. patent application Ser. No.08/418,640, (now U.S. Pat. No. 5,667,373) filed Apr. 7, 1995, assignedto the assignee of the present invention, discloses synthetic linesystems that can be adapted for this purpose. Furthermore, frames of lowintensity ultrasound may be interleaved with high intensity frames.

Multiple transmit zone firing (compound focus) may also yield a betterresult by spreading the beam between two selected focal regions. Thesetechniques can be implemented on the transmit beamformer described inU.S. patent application Ser. No. 08/286,652, as described above.

As discussed above, ultrasonic energy responsive to any of the varioustransmit waveforms is transmitted substantially simultaneously along anultrasound line with two or more focal regions. For example, transmitwaveforms with Gaussian envelopes and associated with different focalregions are summed. As discussed above, the waveforms preferably havethe same fundamental center frequency. The summed signal excites thetransducer to generate a beam of ultrasonic energy focused at two ormore depths. The energy reflects from structures in the target, such astissue and any contrast agent included in the tissue.

In alternative embodiments, two transmit beams are transmittedsubstantially simultaneously along two different scan lines. Forexample, one transmit beam is focused at a depth along one scan line,and another transmit beam is focused at the same or a different depthalong a parallel scan line with a different origin, a scan line at adifferent angle or a scan line at a different angle with a differentorigin. Transmit beams are focused along different scan lines bychanging the delays associated with waveforms at each transducerelement. In this embodiment, each transmit beam may be associated withthe same fundamental center frequency. Alternatively, differentfundamental center frequencies are transmitted substantiallysimultaneously for each transmit beam. In this embodiment, thefundamental center frequencies and corresponding bandwidths arepreferably chosen to minimize transmission for any transmit beam at aharmonic of any of the fundamental frequencies.

When multiple transmit beams are simultaneously transmitted and focusedalong different scan lines for fundamental imaging, acoustic clutter mayremain. When used for harmonic imaging, the clutter may be reduced.

Also as discussed above, the transmit beamformer of U.S. applicationSer. No. 08/286,652 is operable to generate and sum waveformscorresponding to two or more focal depths along the beam. A desired timedomain signal or excitation waveform is-divided by the carrier to obtainthe desired complex envelope using complex values for both the timedomain signal and the carrier. The complex envelope is sampled at arelatively low frequency for generating the excitation waveforms. Thewaveforms are summed to generate an ultrasound beam with two or morefocal points (i.e. two or more transmit beams along a same scan line).

By way of example, the parameters provided for waveforms focused at afirst depth in Appendices 1, 3 and 4 may also be used for each of thetwo or more focal depths, except for the number of beams parameter. SeeU.S. Pat. No. 5,675,554. The number of transmit beams is set to 2 ormore and corresponds to the number of focal points. Each transmit beamcorresponds to a different focal depth along the same transmit or scanline (i.e. two transmit beams correspond to transmission of a transmitbeam focused at two focal depths) and at the same fundamental centerfrequency.

As discussed above, harmonic energy at the second harmonic is imaged.Using any of various receive beamformers, such as the receive beamformerdescribed in U.S. patent application Ser. No. 08/432,615 (see U.S. Pat.No. 5,685,308), ultrasonic energy from the target is received at aharmonic of the fundamental frequency. Other beamformers may be used.See Appendix 2 for various parameters associated with harmonic energyreception. Other parameters may be used. The number of receive beams is2 in this example. For two focal depths, two or more receive beams maybe used, each using the parameters disclosed in Appendix 2. For example,two receive beams corresponding to a same scan line and the two focaldepths of a transmission are used. Alternatively, two or more receivebeams for two or more transmit beams along different scans lines areused. In yet another alternative, multiple receive beams correspondingto different scan lines are used for each single transmitted beam (i.e.synthetic lines as discussed above).

Compound focus transmission is performed with simultaneous transmissionto two or more focal regions, as discussed above, or with sequentialtransmission to each of the two or more focal regions. In oneembodiment, a same aperture for each focal point is used forsimultaneous transmission to two focal points. For a third or fourthfocal region, a waveform corresponding to the focal regions is summedwith the other waveforms as discussed above. The contribution to thesummed signal from one waveform may be negligible for some elements inthe aperture, such as at the end of a linear array when one of the beamsis transmitted with a small active aperture.

For sequential compound focus, the number of simultaneous transmit beamsis preferably 1, but may be 2 or more. The number of simultaneousreceive beams may be 1 or more. A first transmit beam corresponding to afirst focus depth is transmitted. Harmonic echoes responsive to thefirst transmit beam are received along a portion of or the entire depthof the scan line. A second transmit beam corresponding to a second focusdepth is then transmitted. Harmonic echoes responsive to the secondtransmit beam are received along a portion of or the entire depth of thescan line. Both sets of transmit and receive beams are along the samebeam path or scan line and at the same fundamental center frequency. Anyecho information from both transmit beams representing the same range iscombined or one value is selected.

For additional focus depths, additional sets of transmit and receivebeams are processed sequentially with the sets discussed above. Forexample, each of four sets corresponds to 20, 40, 60 and 90 mm focaldepths. To image an area of the target, a frame of data representing thearea is obtained. The frame of data corresponds to a plurality of scanlines. Each scan line corresponds to two or more focus depths.

Preferably, the target is free of contrast agent throughout an entireimaging session as discussed below. Alternatively, contrast agents areinjected during the imaging session.

A same aperture for each focal point may be used for sequentialtransmission to two or more focal points. For two focal points, twowaveforms are sequentially generated for each transducer element in theaperture. In this example, the sequential transmissions uses the sameaperture width with different delay profiles for each focal point.Alternatively, a different aperture width may be used for each focalpoint.

In alternative embodiments, the various foci associated with depths andlines are combined in any of the embodiments discussed above. Forexample, a transmit beam focused at a depth or discrete point and atransmit beam having a line focus are transmitted. The beams aretransmitted sequentially or simultaneously along the same or differentscan lines. In one embodiment, the beams are transmitted along the samescan line. The single depth may overlap with the line focus or occurbefore or after the near or outer focal limits, respectively, of theline focus. The depths discussed above correspond to ranges from thetransducer. Furthermore and as discussed above, the fundamental centerfrequencies associated with each transmit beam may be the same ordifferent.

In yet another alternative embodiment, compound focus of two or morebeams each having a line focus is used. Each transmit beam may betransmitted sequentially or simultaneously, at the same or differentfundamental center frequency, and along the same or different scanlines. For transmit beams along the same scan line, the line focus forone beam may overlap the line focus for another beam. For example, thenear and/or outer limit of one line focus occurs between the near andouter limits for another line focus. Alternatively, the lines do notoverlap (i.e. the outer limit of a line focus closer to the transduceroccurs at a depth before the near limit of another line focus).

Compound focus provides a spreading or distribution of higher intensitythrough a greater depth of field or over a wider region. A rapidevolution of field intensity, coupled with remaining at a highintensity, may result in more effective harmonic imaging. Compound focusmay provide a peak intensity, such as the maximum allowed MechanicalIndex, for multiple points in a scan region or along a scan line ascompared to one point associated with a single focal point.

Compound focus or other transmission techniques discussed herein may beused with various types of waveforms, such as Gaussian waveforms. It hasbeen discovered that bipolar waveforms derived from switched DC levelscan be filtered to provide effective suppression of ultrasonic energy atthe harmonic. This can represent a substantial advantage, becausebipolar, switched DC pulse sequences can be generated in a relativelyinexpensive beamformer.

FIG. 10 shows a block diagram of one channel of a transmit beamformer200 that utilizes such bipolar switched DC transmit waveforms. As shownin FIG. 10, each channel of the transmit beamformer 200 includes abinary waveform generator 202 that responds to inputs from an inputclock, a select line defining the number of waveform cycles to beincluded in each pulse of energy, and a start input from a focal delaymemory 204 that is in turn responsive to an acoustic line select input.Once the appropriate acoustic line has been selected, the focal delaymemory 204 provides a start signal to the binary waveform generator 202at the appropriate time, and the binary waveform generator 202 generatestwo output waveforms, which are 180° out of phase in this example.

These two waveforms are applied to respective pulse amplifiers 206, 208.Each of the pulse amplifiers 206, 208 responds to a high voltage inputwhich can be switched to one of a plurality of levels such as 10, 20 and50 volts in this example. The pulse amplifiers 206, 208 operate withpositive and negative high voltage rails, respectively. Typically, eachpulse amplifier 206 includes a transistor circuit used to switch thehigh voltage through the pulse amplifier 206, 208 to the output of theamplifier 206, 208. Each pulse amplifier outputs an amplified binarywaveform, and the waveforms are summed at a summing node prior to beingapplied as an input to an analog lowpass filter 210. The signal appliedas an input to the analog lowpass filter is a bipolar, switched DCtransmit waveform. The output of the analog lowpass filter 210 is afiltered bipolar transmit waveform, which is applied to an ultrasonictransducer element 214 via a protection circuit 212. The protectioncircuit 212 is included to protect the receive electronics from highvoltage transmit pulses. This circuit is typically based on a diodeclamping circuit. Returning ultrasonic echoes from the target areconverted by the transducer element 214 into electrical signals that arepassed via the protection circuit 272 to a receiver (not shown), thattypically includes a receive beamformer.

The analog lowpass filter 210 may be of any standard type. The filter210 may comprise various resistors, inductors, and capacitors, and mayinclude Bessel, Butterworth or Chebyshev filters, for example.Preferably, the filter 210 achieves approximately 20 dB of suppressionat the second harmonic (two times the fundamental frequency). Ideally,the filter 210 may achieve a far higher degree of suppression. This maybe achieved by using a high order filter, or a high suppression filtersuch as a Chebyshev filter. If desired, a multipole filter or a bandpass filter can be used. In some applications, the components of theanalog lowpass filter 210 are made switchable so that the cutofffrequency of the filter 210 can be varied according to the frequencycharacteristics of the transducer element 214 being used. This switchingmay be accomplished using relays or semiconductor analog switches suchas those supplied by Supertex as HV2 series type switches.

Although the filter 210 has been described above with respect to passivecomponents, it should be understood that the filter 210 may beimplemented using active filter circuits, including operationalamplifiers for example. Such active filter circuits are well-known inthe art, and are described for example in Horowitz and Hill, The Art ofElectronics, Chapter 4 (Cambridge University Press, 1984).

Unipolar pulse sequences can also be formed using circuitry similar tothat described in Maslak U.S. Pat. No. 4,140,022, which provides pulseshaving controllable amplitude, period, and pulse count. See also thedescription of pulse sequence transmitters in IEEE 1980 UltrasonicSymposium, pp. 757-762 (Karrer, et al. "A Phased Array Acoustic ImagingSystem for Medical Use").

It is not required in all embodiments that the lowpass filter operate onthe high voltage transmit waveform. An alternative beamformer 220 (FIG.11) sums the two chopped DC output signals of a binary waveformgenerator 222 at a summer 223, and applies this summed signal as aninput to a lowpass filter 224. The output of the lowpass filter 224 isapplied to an adjustable gain amplifier 226 which amplifies the filteredsignal to a voltage level appropriate for application to the transducerelement 230 via the protection circuit 228. This approach allows thewaveform to be generated and filtered at a relatively low voltage. Thisallows components rated at a lower voltage to be used in the filter 224,which may provide an important cost reduction.

As another alternative, the lowpass filter 210, 224 may include twocomponents, one for each of the unipolar components of the transmitwaveform prior to the point in the signal path where they are summed.See FIG. 11a, where the two components of the lowpass filter are shownat 232, 234

FIG. 12 provides waveforms that illustrate the operation of the transmitbeamformers 200, 220. As shown in FIG. 12, the waveforms 240, 242represent the output waveforms from the binary waveform generator 202,222. Note that the positive excursions of the waveform 240 are out ofphase by 180° with respect to the negative excursions of the waveform242. The waveforms 240, 242 are best considered as unipolar componentsof the bipolar transmit waveform. The summation of the waveforms 240,242 is shown at waveform 244. This summed waveform is bipolar in that itincludes both positive and negative components, and is generated asdescribed above as a sum of switched DC signals. The waveform 244represents the bipolar transmit waveform that is applied as an input tothe analog lowpass filters 210, 224.

FIGS. 13 and 14 provide further information regarding the unfilteredbipolar transmit waveform 244. As shown in FIG. 13, this waveformdisplays excursions from +1 to -1 in an arbitrary amplitude scale, andit includes four cycles at a frequency of 2.5 MHz. FIG. 14 shows thefrequency spectrum 246 of the bipolar transmit waveform 244 of FIG. 13.Note that maximum ultrasonic power is exhibited at the fundamentalfrequency (2.5 MHz), and that there is substantial power reduction(greater than -30 dB) at harmonics of the fundamental, such as 5 and 7.5MHz.

FIGS. 15 and 16 are corresponding graphs for the filtered bipolartransmit waveform supplied as an output of the analog lowpass filter210, 224 of FIGS. 10 and 11 and the summer 223 of FIG. 11 a. Thisfiltered bipolar transmit waveform is identified as reference numeral248, and includes four cycles with a fundamental frequency of 2.5 MHz.As shown in FIG. 16, the frequency spectrum 250 of the waveform 248 hassubstantially reduced power (by greater than 30 dB) at all frequenciesgreater than about 3.6 MHz.

FIG. 17 provides a filter transfer function for a five pole Butterworthfilter (F_(C) =3.75 MHz) that can be used for the lowpass filter 210,224. This transfer function 252 reduces ultrasonic power at the harmonic5 MHz with respect to the fundamental 2.5 MHz by more than 10 dB, and byabout 13 dB in this example.

Of course, it will be understood by those skilled in the art that thevoltage applied to the transducer elements in the beamformers 200, 220,220' of FIGS. 10-11a described above will typically also be a functionof transducer element number. Typically, apodization is employed so thatend transducer elements are operated at a lower voltage than are thecenter transducer elements. This may be achieved by switching the highvoltage input to the pulse amplifiers 206, 208 or by adjusting the gainselect input for the amplifier 226 to different voltage levels accordingto the element position of the transducer element in the array. Forexample, the adjustable amplifier 226 can be a voltage controlledamplifier and the voltage control can be determined by apodizationrequirements.

FIGS. 18-23 relate to switched filter circuits that may be used in amethod or a system for ultrasonic imaging in which ultrasonic energy istransmitted at a fundamental frequency to a target, and reflectedultrasonic energy at a harmonic of the fundamental frequency is receivedfor imaging.

As shown in FIG. 18, the filter circuit 300 defines two signal paths.The first signal path proceeds from the input 302 via the conductor 306to the output 304. The second signal path proceeds from the input 302via the conductor 308 to the output 304. In this embodiment the secondsignal path includes a filter 312 and a isolation circuit 314.

The input 302 is intended to be coupled to the output of a ultrasonictransmitter, which can operate to generate unipolar, bipolar or shapedwaveforms having substantial energy at the fundamental frequency. Theoutput 304 of the filter circuit 300 is intended for connection eitherdirectly or indirectly to an ultrasonic transducer. For example, theoutput 304 may be coupled (either directly or indirectly) to atransducer connector or to a transducer cable. Similarly, the output 304may be coupled to the input of an ultrasonic receiver as shown in FIG.18, to a receiver protection circuit (not shown) or to atransmit/receive switch (not shown).

A switch 310 is controlled to select either the conductor 306 or theconductor 308. In a first mode of operation the switch 310 is positionedas shown in FIG. 18, and the transmitter is coupled to the transducervia the first signal path including the conductor 306. In this mode ofoperation the filter 312 is by-passed. In a second mode of operation theswitch 310 can be placed by the control in connection with the conductor308, and transmit signals from the transmitter to the transducer thenpass through the filter 312 and the isolation circuit 314.

The filter 312 can be any suitable filter designed to suppress energy atthe harmonic of interest. For example, the filter 312 can be a bandreject filter centered about the second harmonic (or other desiredharmonic such and third harmonic) of the fundamental frequency.Alternately, the filter 312 can be a bandpass filter centered on thefundamental frequency or a low pass filter.

The isolation circuit 314 is not required in all embodiments. However,it provides the advantage of avoiding electrical loading of thetransducer elements or of the receiver input stage. Such loading isacceptable, but better performance may be obtained using the isolationcircuit 314 in many cases.

The filter circuit 320 of FIG. 19 is similar to that of FIG. 18, excepttwo switches 330, 331 are provided, operated in parallel by the control.When the switches 330, 331 are positioned as shown in FIG. 19, the input322 is connected via the conductor 326 to the output 324, and both thefilter 332 and the isolation circuit 334 are completely isolated fromthe signal path. When the switches 330, 331 are in the oppositeposition, in which the selected signal path includes the conductor 328,signals passing from the transmitter to the transducer are passedthrough the filter 332 on the isolation circuit 334.

In FIG. 19 the receiver input is connected adjacent the input 332, andthe switches 330, 331 can automatically be controlled such that they areplaced in the state shown in FIG. 19 during receive operations and areautomatically switched to the opposite state for transmit operations.

The filter circuit 340 of FIG. 20 is similar to the circuit 300 of FIG.18, with two exceptions. In this case the switch 350 has been placedadjacent the input 342, and the isolation circuit 354 has been movedadjacent the output 344. Thus, signals pass through the isolationcircuit 354 regardless of whether the switch 350 routes transmit pulsesvia the conductor 346 or via the filter 352 and the conductor 348.

As shown in FIG. 21, a filter circuit 360 can be provided which providestwo signal paths including conductors 366 and 368, respectively, betweenan input 362 and an output 364 for each of 128 channels. The followingdiscussion refers only to channel 1, but the other channels areconfigured similarly. In this case the filter circuit 360 includes a 128channel 1:2 multiplexer 370 that switches the transmitter signal appliedto the input 362 either to output terminal 1 (connected to conductor366) or to output terminal 129 (connected to filter 372, isolationcircuit 374 and conductor 368). Thus, in a first mode of operation ofthe multiplexer 370, transmitter output signals on each of 128 channelsare routed via the respective inputs 362 and the respective conductors366 to the respective outputs 364, thereby bypassing the filters 372 andthe isolation circuits 374. In a second mode of operation as selected bythe control, transmit signals are routed from the inputs 362 via therespective filters 372, isolation circuits 374, and conductors 368 tothe respective outputs 364, thereby placing the filters 372 and theisolation circuits 374 in the signal paths.

In any of the embodiments of FIGS. 18-21 the isolation circuits can beeliminated, they can be placed as shown in FIG. 20 in a common portionof the first and second signal paths, or they may be placed as shown inFIGS. 18, 19 and 21 in only one of the signal paths. Also as shown inFIGS. 18-21 the receiver can if desired be connected to the combinedsignal paths either upstream or downstream of the filter. In the eventthe receiver is connected upstream of the filter as shown in FIG. 19,the switch or switches should be controlled to prevent the filter fromoperating in the receive mode, at least in those cases where thereceiver is responsive to harmonic ultrasonic energy. In cases such asFIG. 18 where the receiver is connected to the signal path downstream ofthe filter, the switch 310 can be controlled to bypass the filter 312selectively, as for example when the transmitter is used in analternative mode of operation in which both the transmitter and thereceiver are centered at the same fundamental frequency. As shown inFIG. 21, the switches for the filters may be part of a pre-existingtransducer multiplexer.

FIG. 22 is a schematic diagram of a filter circuit 380 that is similarto the filter circuit 320 of FIG. 19 with two exceptions. First, thefilter circuit 380 does not include an isolation circuit. As explainedabove, such isolation circuits are optional. Second, the filter circuit380 includes a total of three signal paths 386, 388, 389 between theinput 382 and the output 384. The switches 390, 391 route signals fromthe transmitter to the output via anyone of these three signal paths.The signal path 386 includes no filter while the signal paths 388, 389include separate respective filters 392, 393. The filters 392, 393 mayfor example be band pass filters appropriate for two differentfundamental frequencies. In some applications, the unfiltered signalpath 386 can be eliminated.

The filter circuit 400 of FIG. 23 includes a filter circuit 406interposed between an input 402 and an output 404. In this case thefilter circuit 406 includes component filters 408, 410, and a switch 412is used to connect the signal path containing the desired one of thecomponent filters 408, 410 to the filter circuit 406. In this way, thefilter characteristics of the filter circuit 406 can be adjusted asappropriate for the particular application by controlling the switch 412to select the appropriate one of the filters 408, 406. As used herein,the term "filter" is intended broadly to encompass both filter circuitssuch as low pass, band pass, or band blocking filter circuits as wellfilter components such as capacitors, inductors and other electricalcomponents that affect the filter characteristics of a filter circuit.

Switched filters similar to any of those shown in FIGS. 18-23 may beused in the systems shown in FIGS. 2, 8, 9, 10, 11 and 11a. For example,switched filters (using any of the approaches described above) may besubstituted for the unswitched filter 58 of FIG. 2, the filter 104 ofFIG. 8, the filter 124 of FIG. 9, the filter 210 of FIG. 10, the filter224 of FIG. 11, or the filters 232, 234 of FIG. 11a.

As an alternative or in addition to the filtering circuits discussedabove, other circuitry may be used to tune each transducer element fortransmit and receive processing. This other circuitry tunes thetransducer element to pass frequencies in a desired band, such asfundamental frequencies for transmit or harmonic frequencies for receiveprocessing. Other frequencies, such as frequencies outside the passband, may be suppressed by the tuned transducer frequency response.Preferably, the frequency response of the transducer element is adjusteddifferently for transmit and receive processing.

Referring to FIG. 24, a tuning circuit is generally shown at 500. Thecircuit 500 includes a transducer element 502 connected in parallel to aswitch 504 and an inductor 506. A transmit waveform input 508, anisolation circuit 510 and an inductor 512 connect in series with thetransducer element 502. A diode protection clamp 514, a low voltagepre-amplifier 516 and a receive signal output 518 also connect to thetransducer element 502.

Any of various transducers and associated transducer elements 502 may beused. In one embodiment, an Acuson C3 curved linear radiology transduceris used. This transducer has a 3.5 MHz center frequency without tuning(i.e. without the tuning circuitry shown in FIG. 24). The C3 transducercenter frequency is appropriate for use in harmonic imaging, since lowertransmit frequencies, such as 2-3 MHz, may be more effective forharmonic processing. Furthermore, the impedance characteristics of theC3 transducer may be more easily adjusted by the tuning circuit 500 thanother transducers. Other transducers may be used.

The inductors 506 and 512 are used to tune the transducer. The inductorsare of any impedance and may be determined experimentally. In oneembodiment, the inductor 512 is 7.5 μH, and the inductor 506 is 1.5 μH.

The transmit waveform is input and passes through the inductor 512. Fortransmission, the switch 504 is open. In one embodiment, the switch 504comprises an analog switch with digital control, such as the SupertexHV2 series switch (Supertex Inc., Sunnyvale Calif.). For reception, theswitch 504 is closed. The receive signal is amplified by the amplifier516 and is prevented from passing to the transmit beamformer by theisolation circuit 510.

Referring to FIG. 26, the frequency response associated with the C3transducer without tuning is shown by a dashed line 520. By passing thetransmit waveform through the inductor 512, the frequency responseduring transmit is tuned as shown by a solid line 522. By switching theinductor 506 into a parallel connection with the transducer element 502,the frequency response during reception is tuned as shown by a dash-dotline 524.

The transmit tuning passes lower frequencies, such as associated withtransmission at a 2.0 MHz fundamental center frequency, and filters thetransmitted waveform at the harmonic frequencies, such as at 4.0 MHz, ascompared to the un-tuned frequency response (see dashed line 520). Thereception tuning passes higher frequencies, such as associated with the4.0 MHz second harmonic center frequency, and filters the receivedwaveform at fundamental frequencies, such as 2.0 MHz, as compared to theun-tuned frequency response (see dashed line 520).

To change the relative sensitivity of the transducer at differentfrequencies, the value of the inductors 506 and 512 are changed to havegreater or lesser inductance.

Referring to FIG. 25, an alternative embodiment of a tuning circuit isgenerally shown at 600. The circuit 600 includes a transducer element602 connected in parallel through a cable 620 and a switch 604 to aninductor 606. A transmit waveform input 608 and an inductor 612 connectin series with the transducer element 602 through the switch 604 andanother switch 622. A receive signal output 618 connects to the inductor606 through the switch 622. The receive signal output 618 providessignals to additional circuitry, such as a diode clamp andpre-amplifier.

In operation, the transducer is tuned for transmission by connecting theinductor 612 in series with the transducer element 602 through switches604 and 622. The transducer is tuned for reception by connecting theinductor 606 in parallel with the transducer element 602 with switches604 and 622.

In alternative embodiments, tuning is provided in only transmit or onlyreceive. For example, if only transmit tuning is provided, the switch504 and the inductor 506 are removed. The transducer may be selected fora desired receive frequency response and tuned for a desired transmitresponse. Other tuning components, such as resistors, differentinductors, capacitors, transformers or combinations thereof may be usedto tune the transducer through parallel, series or combined networkconnections.

In the systems described above, bi-polar rectangular or uni-polartransmit waveforms may also be shaped. The amplitude of each transmitwaveform is shaped to gradually rise to a maximum value and graduallydecrease from the maximum value. Each transmit waveform is shaped bymodulating a carrier waveform with an envelope waveform. Alternativelyand with respect to transmitting a uni-polar waveform, a shaped low passoff-set waveform with a gradually increasing and decreasing amplitude issummed with a bi-polar waveform in real-time or off-line. Shaping theamplitude of bi-polar rectangular or uni-polar transmit waveforms toreduce energies associated with harmonic frequencies is discussed inU.S. application Ser. No. 08/893,150 (now U.S. Pat. No.5,913,813)(Attorney Docket No. 5050/220) for Ultrasonic Imaging MethodAnd System For Transmit Signal Generation For An Ultrasonic ImagingSystem Capable Of Harmonic Imaging, assigned to the assignee of thepresent invention and filed concurrently herewith, the disclosure ofwhich is hereby incorporated by reference.

As yet another alternative embodiment, the transmit waveforms discussedabove may be pulse width modulated. As disclosed in U.S. applicationSer. No. 08/893,287 (now U.S. Pat. No. 5,933,614) (Attorney Docket No.5050/218) for Ultrasonic Imaging Method and Apparatus For GeneratingPulse Width Modulated Waveforms With Reduced Harmonic Response, assignedto the assignee of the present invention and filed concurrentlyherewith, the disclosure of which is hereby incorporated by reference,the duration of each pulse within a burst is selected to reduce theenergy transmitted at harmonic frequencies. In particular, the durationof one or more pulses is different than other pulses within the burst.Preferably, the widths of the pulses within the burst gradually increaseand then decrease, but other duration patterns may be used.

As yet another alternative embodiment, the transmit waveforms discussedabove may be shaped as a function of summation of the waveforms in theacoustic domain. As disclosed in U.S. application Ser. No. 08/893,271abandoned (Attorney Docket No. 5050/219) for Ultrasonic Imaging MethodAnd System For Harmonic Imaging Pulse Shaping In The Acoustic Domain,assigned to the assignee of the present invention and filed concurrentlyherewith, the disclosure of which is hereby incorporated by reference,the transmit waveform associated with a first transducer element orelements is shaped relative to a second waveform associated with asecond transducer element or elements. For example, the first waveform(1) is delayed by a fraction of a cycle or one or more cycles, (2) isadjusted in amplitude, (3) is transmitted for a different number ofcycles or (4) any combination of two or all three of (1), (2), and (3)relative to the second waveform. The first and second waveforms arefocused at a point and transmitted. The transmitted waveforms sum in theacoustic domain at the point to form the desired waveform for reductionof energies transmitted in the harmonic frequencies. Preferably, thedesired waveform corresponds to an amplitude that rises gradually to amaximum value and decreases gradually from the maximum value.

As an example of generating transmitted waveforms as a function of theresulting summed signal in the acoustic domain, the first and secondwaveforms are rectangular waves. The first waveform is delayed, inaddition to any focusing delay, by may be 1/2 of a cycle relative to thesecond waveform. At the point in the body, the first and second waveformsum together to form a third waveform. The third waveform has threeamplitude levels (0,1,2). The greatest amplitude is associated with anoverlap of the first and second transmit waveforms and is preferably atthe center of the third waveform. The number of cycles and amplitudeshape of the first and second waveforms may also be controlled to createthe desired third waveform in the acoustic domain.

Any of the various alternatives discussed above, such as pulse widthmodulation, filtering, generation of waveforms with multiple amplitudesand summation of waveforms in the acoustic domain, may be used incombination. The combination may include more than two of thealternatives discussed above.

All of the harmonic imaging techniques described above can be used inboth tissue and contrast agent harmonic imaging modes. In the tissueharmonic imaging mode, no additional non-linear contrast agent is addedto the target, and only the non-linear characteristics of the tissue arerelied on to create the ultrasonic image. Medical ultrasound imaging istypically conducted in a discrete imaging session for a given subject ata given time. For example, an imaging session can be limited to anultrasound patient examination of a given tissue of interest over aperiod of 1/4 to 1 hour, though other durations are possible. In thiscase no additional non-linear contrast agent is introduced into thetissue at any time during the imaging session. In the contrast agentharmonic imaging mode, any one of a number of well known non-linearcontrast agents such as those described above can be added to the targetin order to enhance the non-linear harmonic response of the tissue. Forthis reason, it should be understood that the introduction of an addednon-linear contrast agent into the tissue being imaged is not implied inany of the following claims unless such added non-linear contrast agentis expressly recited.

From the foregoing, it should be apparent that improved systems andmethods for imaging contrast agent have been disclosed. These systemscan use the transmit beamformers described in U.S. Pat. No. 5,608,690,which focuses different frequency components at different ranges. Also,as described above, these techniques can be used with other beamformerswhich utilize other transmit waveforms. Of course, the various aspectsof this invention relating to harmonic suppression and line focus (orother spreading of the region of maximum intensity) can be usedseparately from one another, rather than in combination as describedabove. It is intended that the foregoing detailed description beregarded as illustrative rather than limiting. It is the followingclaims, including all equivalents, which are intended to define thescope of this invention.

                                      APPENDIX 1                                  __________________________________________________________________________    Parameters for Transmit Beamformer                                            (all references are to FIG. 3 of U.S. Pat. No. 5,675,554)                     Parameter                                                                            Corresponds to                                                                            Value                                                      __________________________________________________________________________    h2     FIR filter h2 of T312                                                                     (14641)                                                    h3     FIR filter h3 of T324                                                                     (10-201)                                                   Ku2    Upsampler of T326                                                                         2                                                          Ku1    Upsampler of T312                                                                         2 (determined by: KUI = Nb*4/Ku2)                          h4     FIR filter h4 of T326                                                                     (232)                                                      Nb     Number of transmit beams                                                                  1                                                          cw.sub.-- on       0 -> pulsed mode                                           Envelope Type,                                                                       `Envelope` of FIG. 4                                                                      0 -> real                                                  beam 0                                                                        Ns                 B-mode: 17                                                                    F-mode: 23                                                 h1     Envelope in T304                                                                          B-mode: (0.0039, 0.0156, 0.0430,                                              0.1133, 0.2461, 0.4531, 0.7031, 0.9141,                                       0.9961, 0.9141, 0.7031, 0.4531, 0.2461,                                       0.1133, 0.0430, 0.0156, 0.0039)                                               (33% Gaussian, sampled at Fe =  10 MS/s)                                      F-mode: (0.0039, 0.0078, 0.0195,                                              0.0430, 0.0898, 0.1719, 0.2930, 0.4531,                                       0.6406, 0.8203, 0.9492, 0.9961, 0.9492,                                       0.8203, 0.6406, 0.4531, 0.2930, 0.1719,                                       0.0898, 0.0430, 0.0195, 0.0078, 0.0039)                                       (25% Gaussian, sampled at Fe = 10 MS/s)                    phi    Phase to be applied at T310                                                               See definition of phi below                                Fs     Sampling Freq at                                                              O/P of T328 40 MS/s                                                    Fe     Sampling Freq of                                                              envelope in T304                                                                          10 MS/s (determined from above based on: Fe                                   = Fs/Ku1/Ku2                                               Fpa select                                                                           v.sub.-- phi = Fc/F0                                                                      0 -> use modulation frequeency for focusing                Fm/F0  v = Fc/F0   0.5 (Fm = 2.5 MHz; F0 = 5.0 MHz)                           FO                 5.0 MHz                                                    Together these terms, plus the fact that the envelope is real, are used       to calcuiate the phase                                                        applied in complex multiplier T310, as described in Application Ser. No.      08/432,056. In                                                                particular, this phase is broken into:                                        envelope phase: zero because envelope is real                                 fine focusing: calculated from the difference between quantized and ideal     delay, using the                                                              phase alignment frequency vphi = v                                            modulation: complex multiplier T310 includes a component which                corresponds to                                                                modulation to a frequency (Fm/F0 - 1)*F0. This modulation, in combination     with later                                                                    modulation in complex multiplier T318, results in an overall effective        modulation frequency Fm.                                                      phi = phi.sub.-- D + phi.sub.-- E = phi.sub.-- R                              phi.sub.-- D                                                                      (Phase portion of delay (fine focusing)) = -2.(pi)v.sub.-- phiτsub        .-- phi                                                                       tau.sub.-- phi = low order portion of the delay word representing             fractional units of                                                           T0 (1/F0) as in Pat. No. 5,675,554. This is the portion of the                specified                                                                     focusing delay which is applied via phasing rather than true time             delay.                                                                    phi.sub.-- E = 0 (Waveform sample phase is zero because envelope is           real)                                                                         phi.sub.-- R = 2.(pi).kul.(v - 1).n/4 (n is the successive sample             number)                                                                       This is a phase rotation of 2.(pi).(Fm - F0).t where t = kul.t/(4.F0)         __________________________________________________________________________

                                      APPENDIX 2                                  __________________________________________________________________________    Parameters for Receive Beamformer                                             (all references are to FIG. 3 of U.S. Pat. Application Ser. No.               08/432,615)                                                                   Parameter                                                                             Corresponds to                                                                            Value                                                     __________________________________________________________________________    Fs      Sampling rate Fs at ADC                                                                   40 MS/s                                                   Nb      Number of receive beams                                                                   2                                                         Kd1     Downsampler of R162                                                                       2                                                         h1      FIR filter hl of R160                                                                     (232) (selected based on Kd1)                             h1 bypass           disabled                                                  h2      FIR filter h2 of R164                                                                     (10-1)                                                    h2 bypass           disabled                                                  h3      FIR filter h3 of R167                                                                     (14j-8-10j84j-1)                                          h3 bypass           disabled                                                  Kd2     Downsampler of R169                                                                       4 (determined by: Kd2-Nb*4/Kd1)                           Fb                  5 MS/s (determined from above, based                                          on Fb = Fs/Kd1/Kd2)                                       Fp select           0 -> Fp = Fstart                                          Fp select           1 -> Fp = Fstart - Fdownslope*R                           Fstart/FO                                                                             Fstart      B-mode: Fstart/FO = 1.1172 (5.5859 MHz)                                       F-mode: Fstart/F0 = 1.0781 (5.3906 MHz)                   Fdownslope                                                                            (DELTA F sub downshift)                                                                   B-mode: Fdownslope = 1.793E-4 FO/TO                                           <-> 5821 Hz/mm                                                                F-mode: Fdownslope = 1.144E-4 FO/TO                                           <-> 3716 Hz/mm                                            Tbreak              Tbreak >= 1792 TO (276 mm)                                Fupslope            0                                                         Base Band Filter                                                              = = =                                                                         Ntaps               16                                                        type                real                                                      L/M ratio           1/1 (based on L = 1, M = 1)                               hbbf    Base band filter                                                              coefficients                                                                              (based on L = 1, M = 1)                                                       B-mode: hbbf = (0, 0, 0, 0, 0,                                                0.0195, 0.0996, 0.2285, 0.2988, 0.2285,                                       0.0996, 0.0195, 0, 0, 0, 0)                                                   F-mode: hbbf = (0, 0, 0, 0.0020, 0.0137,                                      0.0469, 0.1172, 0.1992, 0.2383, 0.1992,                                       0.1172, 0.0469, 0.0137, 0.0020, 0, 0)                                         Baseband filter is an FIR operating on the                                    output of the receive beamformer                          __________________________________________________________________________

                                      APPENDIX 3                                  __________________________________________________________________________    Revised Parameters for Transmit Beamformer                                    All parameters are as defined in Appendix 1 and are set                       equal to the values of Appendix 1 except as noted.                            H3 = [10 - 1]                                                                 Ns = 17 (B-mode)                                                                 17 (F-mode)                                                                B-mode Envelope =                                                                      [.01171875                                                                           .03125                                                                              .078125                                                                             .16796875                                                                          .32033125                                             .52734375                                                                            .75   .9296875                                                                            .99609375                                                                          .9296875                                              .75    .52734375                                                                           .3203125                                                                            .16796875                                                                          .078125                                               .03125 .01171875]                                                             (29% Gaussian, sampled at Fe = 10 MS/s)                              F-mode Envelope =                                                                      [.015625                                                                             .0390625                                                                            .08984375                                                                           .19140625                                                                          .34375                                                .546875                                                                              .765625                                                                             .93359375                                                                           .99609375                                                                          .93359375                                             .765625                                                                              .546875                                                                             .34375                                                                              .19140625                                                                          .08984375                                             .0390625                                                                             .015625]                                                               (30% Gaussian, sampled at Fe = 10 MS/s)                              __________________________________________________________________________

                  APPENDIX 4                                                      ______________________________________                                        Second Revised Parameters for Transmit Beamformer                             (Adapted for use with Acuson 3V2C Probe)                                      All parameters are as defined in Appendix 1 and are set                       equal to the values of Appendix 1 except as noted.                            H3 = [10 - 1]                                                                 Ku2 = 4                                                                       Ku1 = 1                                                                       Ns = 27                                                                       Envelope =                                                                             [0.015625 0.03125   0.0546875                                                                             0.08984375                                        0.140625  0.21484375                                                                              0.30859375                                                                            0.41796875                                        0.546875  0.6796875 0.80078125                                                                            0.90625                                           0.97265625                                                                              0.99609375                                                                              0.97265625                                                                            0.90625                                           0.80078125                                                                              0.6796875 0.546875                                                                              0.41796875                                        0.30859375                                                                              0.21484375                                                                              0.140625                                                                              0.08984375                                        0.0546875 0.03125   0.01562500]                                             (33% Gaussian, sampled at Fe = 14 MS/s)                                Fs = 56 MS/s                                                                  Fe = 14 MS/s                                                                  F0 = 3.5 MHz                                                                  ______________________________________                                    

We claim:
 1. A method for ultrasonically receiving data for imaging a target with distributed energy transmissions, said method comprising the following steps:(a) transmitting ultrasonic energy centered at a fundamental frequency and focused at a first depth along a scan line; (b) transmitting substantially simultaneously with step (a) ultrasonic energy centered at the fundamental frequency and focused at a second depth along the scan line; and (c) receiving ultrasonic energy from the target at a harmonic of the fundamental frequency, the ultrasonic energy associated with at least the first and second depths.
 2. The method of claim 1 wherein said target is free of contrast agent throughout an entire imaging session.
 3. The method of claim 1 wherein said target comprises tissue and contrast agent.
 4. The method of claim 1 wherein the transmission of step (a) is generated with a set of transducer elements and the transmission of step (b) is generated with the substantially same set of transducer elements.
 5. The method of claim 1 further comprising (d) transmitting substantially simultaneously with steps (a) and (b) ultrasonic energy centered at the fundamental frequency and focused at a third depth along the scan line.
 6. The method of claim 1 further comprising (d) tuning a transducer for steps (a) and (b); and wherein steps (a) and (b) comprise transmitting Gaussian waveforms.
 7. The method of claim 1 further comprising (d) tuning a transducer for step (c).
 8. The method of claim 7 further comprising (e) tuning the transducer for steps (a) and (b), wherein step (e) is operative to enhance a first frequency spectrum oriented near the fundamental frequency and step (d) is operative to enhance a second frequency spectrum oriented near the harmonic.
 9. A method for ultrasonically receiving data for imaging a target with distributing energy transmissions, said method comprising the following steps:(a) transmitting ultrasonic energy centered at a fundamental frequency and focused at a first depth along a scan line; (b) receiving ultrasonic energy associated with at least the first depth in the target at a harmonic of the fundamental frequency; (c) transmitting ultrasonic energy centered at the fundamental frequency and focused at a second depth along the scan line; (d) receiving ultrasonic energy associated with at least the second depth in the target at a harmonic of the fundamental frequency; and (e) performing steps (c) and (d) after steps (a) and (b); wherein the target is free of contrast agent throughout an entire imaging session.
 10. The method of claim 9 wherein the transmission of step (a) is generated with a set of transducer elements and the transmission of step (c) is generated with the substantially same set of transducer elements.
 11. The method of claim 9 further comprising:(f) transmitting ultrasonic energy centered at the fundamental frequency and focused at a third depth along the scan line; and (g) receiving ultrasonic energy associated with at least the third depth in the target at a harmonic of the fundamental frequency.
 12. The method of claim 9 further comprising (f) repeating steps (a)-(e) for a plurality of scan lines.
 13. The method of claim 9 further comprising (f) tuning a transducer for steps (a) and (c); and wherein steps (a) and (c) comprise transmitting Gaussian waveforms.
 14. The method of claim 9 further comprising (f) tuning a transducer for steps (b) and (d).
 15. The method of claim 14 further comprising (g) tuning the transducer for steps (a) and (c), wherein step (g) is operative to enhance first frequency spectrum oriented near the fundamental frequency and step (f) is operative to enhance a second frequency spectrum oriented near the harmonic.
 16. A method for ultrasonically generating data for imaging as a function of a frame of data representing spatial areas in a target, said method comprising the following steps:(a) transmitting a first ultrasonic energy beam centered at a fundamental frequency and focused at a first depth along a scan line; (b) receiving ultrasonic energy associated with at least the first depth in the target at a harmonic of the fundamental frequency; (c) transmitting a second ultrasonic energy beam centered at the fundamental frequency and focused at a second depth along the scan line; (d) receiving ultrasonic energy associated with at least the second depth in the target at a harmonic of the fundamental frequency; (e) performing steps (c) and (d) after steps (a) and (b); (f) repeating steps (a)-(e) for a plurality of scan lines; and (g) generating the frame from data associated with the plurality of scan lines.
 17. The method of claim 16 wherein said target is free of contrast agent throughout an entire imaging session.
 18. The method of claim 16 wherein said target comprises tissue and contrast agent.
 19. The method of claim 16 wherein the transmission of step (a) is generated with a set of transducer elements and the transmission of step (c) is generated with the substantially same set of transducer elements.
 20. The method of claim 16 further comprising (h) transmitting ultrasonic energy centered at the fundamental frequency and focused at a third depth along each scan line.
 21. The method of claim 16 further comprising (h) tuning a transducer for steps (b) and (d); and wherein steps (a) and (c) comprise transmitting Gaussian waveforms.
 22. The method of claim 21 further comprising (i) tuning the transducer for steps (a) and (c), wherein step (i) is operative to enhance a first frequency spectrum oriented near the fundamental frequency and step (h) is operative to enhance a second frequency spectrum oriented near the harmonic.
 23. A method for ultrasonically receiving data for imaging a target with distributed energy transmissions, said method comprising the following steps:(a) transmitting ultrasonic energy centered at a first fundamental frequency in a first transmit beam focused at a first region along a first scan line; (b) transmitting substantially simultaneously with step (a) ultrasonic energy centered at a second fundamental frequency in a second transmit beam focused at a second region along a second scan line, the second scan line different than the first scan line; and (c) receiving ultrasonic energy along at least first and second receive lines from the target at harmonics of the first and second fundamental frequencies, respectively, the first and second receive lines associated with at least the first and second regions, respectively.
 24. The method of claim 23 wherein said target is free of contrast agent throughout an entire imaging session.
 25. The method of claim 23 wherein said target comprises tissue and contrast agent.
 26. The method of claim 23 further comprising (d) tuning a transducer for steps (a) and (b); andwherein steps (a) and (b) comprise transmitting Gaussian waveforms.
 27. The method of claim 23 wherein step (c) comprises forming at least two receive lines associated with each of the first and second scan lines.
 28. The method of claim 23 wherein the second fundamental frequency comprises a same frequency as the first fundamental frequency.
 29. A method for ultrasonically receiving data for imaging a target with distributed energy transmissions, said method comprising the following steps:(a) transmitting first ultrasonic energy centered at a first fundamental frequency in a first transmit beam focused at a first depth; (b) transmitting second ultrasonic energy centered at a second fundamental frequency in a second transmit beam having a line focus; and (c) receiving ultrasonic energy from the target at harmonics of the first and second fundamental frequencies, the ultrasonic energy associated with at least the first depth and with depths associated with the line focus.
 30. The method of claim 29 wherein:step (a) comprises transmitting the first ultrasonic energy focused at the first depth along a scan line; and step (b) comprises transmitting the second ultrasonic energy having the line focus along the scan line.
 31. The method of claim 30 wherein:the line focus comprises an outer focal limit and a near focal limit; and the first depth is at a different depth than depths along the line focus.
 32. The method of claim 31 wherein the first depth is nearer a transducer than the near focal limit.
 33. The method of claim 29 wherein the first and second ultrasound energies are transmitted substantially simultaneously.
 34. The method of claim 29 wherein the first and second ultrasound energies are transmitted sequentially.
 35. The method of claim 29 further comprising step (d) of tuning a transducer; andwherein steps (a) and (b) comprise transmitting Gaussian waveforms.
 36. The method of claim 29 wherein the target is free of contrast agent throughout an entire imaging session.
 37. The method of claim 29 wherein the second fundamental frequency comprises a same frequency as the first fundamental frequency.
 38. A method for ultrasonically receiving data for imaging a target with distributed energy transmissions, said method comprising the following steps:(a) transmitting first ultrasonic energy centered at a fundamental frequency in a first transmit beam having a first line focus; (b) transmitting second ultrasonic energy centered at the fundamental frequency in a second transmit beam having a second line focus; and (c) receiving ultrasonic energy from the target at a harmonic of the fundamental frequency, the ultrasonic energy associated with at least the first and second line foci.
 39. The method of claim 38 wherein:step (a) comprises transmitting the first ultrasonic energy focused along a scan line; and step (b) comprises transmitting the second ultrasonic energy along the scan line.
 40. The method of claim 39 wherein:the first and second line foci comprises first and second outer focal limits and first and second near focal limits, respectively; and the first outer focal limit is at a depth closer to a transducer than the second near focal limit.
 41. The method of claim 39 wherein:the first and second line foci comprises first and second outer focal limits and first and second near focal limits, respectively; and the first outer focal limit is at a depth between the second near and outer focal limits.
 42. The method of claim 38 wherein the first and second ultrasound energies are transmitted substantially simultaneously.
 43. The method of claim 38 wherein the first and second ultrasound energies are transmitted sequentially.
 44. The method of claim 38 further comprising step (d) of tuning a transducer; andwherein steps (a) and (b) comprise transmitting Gaussian waveforms.
 45. The method of claim 38 wherein the target is free of contrast agent throughout an entire imaging session.
 46. A system for ultrasonically receiving data for imaging a target with distributed energy transmissions, the system comprising:means for transmitting ultrasonic energy centered at a first fundamental frequency in a first transmit beam focused at a first region along a first scan line; means for transmitting substantially simultaneously with step (a) ultrasonic energy centered at a second fundamental frequency in a second transmit beam focused at a second region along a second scan line, the second scan line different than the first scan line; and means for receiving ultrasonic energy along at least first and second receive lines from the target at harmonics of the first and second fundamental frequencies, respectively, the first and second receive lines associated with at least the first and second regions, respectively.
 47. The system of claim 46 wherein the first fundamental frequency comprises a same frequency as the second fundamental frequency.
 48. A system for ultrasonically receiving data for imaging a target with distributed energy transmissions, said system comprising:means for transmitting first ultrasonic energy centered at a first fundamental frequency in a first transmit beam focused at a first depth; means for transmitting second ultrasonic energy centered at a second fundamental frequency in a second transmit beam having a line focus; and means for receiving ultrasonic energy from the target at harmonics of the first and second fundamental frequencies, the ultrasonic energy associated with at least the first depth and with depths associated with the line focus.
 49. The system of claim 48 wherein the first fundamental frequency comprises a same frequency as the second fundamental frequency.
 50. A system for ultrasonically receiving data for imaging a target with distributed energy transmissions, said system comprising:means for transmitting first ultrasonic energy centered at a fundamental frequency in a first transmit beam having a first line focus; means for transmitting second ultrasonic energy centered at the fundamental frequency in a second transmit beam having a second line focus; and means for receiving ultrasonic energy from the target at a harmonic of the fundamental frequency, the ultrasonic energy associated with at least the first and second line foci.
 51. A method for ultrasonically receiving data for imaging a target with distributed energy transmissions, said method comprising the following steps:(a) transmitting ultrasonic energy centered at a fundamental frequency in a first transmit beam focused at a first region along a first scan line; (b) transmitting substantially simultaneously with step (a) ultrasonic energy centered at the fundamental frequency in a second transmit beam focused at a second region along a second scan line, the second scan line different than the first scan line; and (c) receiving ultrasonic energy along at least first and second receive lines from the target, the ultrasonic energy centered at a harmonic of the fundamental frequency, the first and second receive lines associated with at least the first and second regions, respectively.
 52. The method of claim 51 wherein said target is free of contrast agent throughout an entire imaging session.
 53. The method of claim 51 wherein said target comprises tissue and contrast agent.
 54. The method of claim 51 further comprising (d) tuning a transducer for steps (a) and (b); andwherein steps (a) and (b) comprise transmitting Gaussian waveforms.
 55. The method of claim 51 wherein step (c) comprises forming at least two receive lines associated with each of the first and second scan lines.
 56. The method of claim 51 wherein the first and second regions correspond to a same depth. 